Since Dennis Gabor, a Hungarian-British physicist invented electron holography in 1947, many advances have been made in the field of electron microscopy. In electron holography, the phase component and the amplitude component of the information are separated by splitting the electron beam in a transmission electron microscope into two parts with a biprism and combining them after at least one part of the beam passes through a sample. By processing the information to reconstruct an object that a part of the beam went through, an image of the sample is reconstructed.
In principle, any physical quantity that affects the phase of an electron beam may be measured and reconstructed by electron holography. Examples of physical quantities that can be measured in electron holography include mechanical features of atoms such as the shapes of nanocrystals or nano-scale voids, electrical field such as ferroelectric materials generate, and magnetic field such as ferromagnetic materials generate. In the semiconductor industry, electron holography has been shown to be valuable in mapping p-n junction characteristics in semiconductor devices, such as a MOSFET, with high spatial resolution. Due to the remarkable advances in this field, commercial electron holography equipments and textbooks on this subject are readily available.
Unlike other applications of transmission electron microscopy, however, electron holography requires an extremely careful sample preparation. The quality of data that can be extracted from a sample depends on the sample preparation method. This is because the phase component of the information used in electron holography is more sensitive to the surface conditions of a sample than other information, such as intensity information, used in other applications of transmission electron microscopy. For example, while regular TEM sample preparation may utilize ion milling, in which energetic ions remove the material while generating some surface damage, to generate a TEM sample of acceptable quality for other TEM applications, the structural degradation of the surface of such samples introduces a significant amount of noise into electron holography images.
Therefore, mechanical polishing has been the method of choice for sample preparation despite the laborious nature of the process. One of the difficulties in the sample preparation is the control of thickness, especially toward the end of the polishing process. Once the sample is overthinned, the sample is no longer suitable for electron holography. According to a commonly used method of preparing a sample for electron holography, the thickness of the sample is checked frequently to insure that overpolishing, and consequently overthinning of the sample does not occur. This requires intensive intervention during the polishing process.
Another difficulty in the sample preparation is that the adhesive material that protects the upper surface of the sample area tends to get rounded during the polishing and also tends to fall apart during the sample preparation.
These difficulties make a sample preparation process for electron holography laborious and unpredictable.
Therefore, there exists a need for a method and structure for self-limiting the thickness of a polished electron microscopy sample, and especially for electron holography sample preparation.
There also exists a need for a method of holding the adhesive material over the top surface of the sample without delamination.
There also exists a need for a method of providing a high quality sample for other transmission electron microscopy methods to provide an enhanced image with better resolution than normal TEM samples.